1. Field of the Invention
The present invention relates to a power transmission system of a hybrid electric vehicle. More particularly, the present invention relates to a power transmission system of a hybrid electric vehicle that can improve fuel economy by achieving electric continuously variable shift at electric vehicle (EV) mode and minimizing the number of frictional elements operated at parallel mode.
2. Description of Related Art
Environmentally-friendly techniques of vehicles are very important techniques on which survival of the future motor industry is dependent. Vehicle makers are focusing on development of environmentally-friendly vehicles so as to meet environment and fuel consumption regulations.
Therefore, the vehicle makers have been developing future vehicles such as electric vehicles (EV), hybrid electric vehicles (HEV), and fuel cell electric vehicles (FCEV).
Since the future vehicles have technical restrictions such as weight and cost, the vehicle makers keep observation upon hybrid electric vehicles for meeting exhaust gas regulations and improving fuel consumption performance and are competing desperately to put hybrid electric vehicles to practical use.
Hybrid electric vehicles are vehicles using more than two power sources, and gasoline engines or diesel engines using fossil fuel and motor/generators driven by electrical energy are mainly used as the power sources of the hybrid electric vehicles.
The hybrid electric vehicle uses the motor/generator having relatively better low-speed torque characteristics as a main power source at a low-speed and uses an engine having relatively better high-speed torque characteristics as a main power source at a high-speed.
Since the hybrid electric vehicle stops operation of the engine using the fossil fuel and uses the motor/generator at a low-speed region, fuel consumption may be improved and exhaust gas may be reduced.
The power transmission system of a hybrid electric vehicle is classified into a single-mode type and a multi-mode type.
A torque delivery apparatus such as clutches and brakes for shift control is not necessary, but fuel consumption is high due to deterioration of efficiency at a high-speed region and an additional torque multiplication device is required for being applied to a large vehicle according to the single-mode type.
Since the multi-mode type has high efficiency at the high-speed region and is able to multiply torque autonomously, the multi-mode type can be applied to a full size vehicle.
Therefore, the multi-mode type instead of the single-mode type is applied as the power transmission system of a hybrid electric vehicle and is also under continuous investigation.
The power transmission system of the multi-mode type includes a plurality of planetary gear sets, a plurality of motor/generators operated as a motor and/or a generator, a plurality of torque delivery apparatus controlling rotation elements of the planetary gear sets, and a battery used as a power source of the motor/generators.
The power transmission system of the multi-mode type has different operating mechanisms depending on connections of the planetary gear sets, the motor/generators, and the torque delivery apparatus.
In addition, the power transmission system of the multi-mode type has different features such a durability, power delivery efficiency, and size depending on the connections of the planetary gear sets, the motor/generators, and the torque delivery apparatus. Therefore, designs for the connection structure of the power transmission systems of a hybrid electric vehicle are also under continuous investigation to achieve robust and compact power transmission systems having no power loss.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.